1. Field of the Invention
This invention relates to cable pulling systems and, more particularly, to a system for drawing a mole through a composition to create a passageway between first and second spaced locations.
2. Background Art
It is well known in the industry to draw a mole through a composition to define a passageway through the composition between first and second spaced locations. It is known to use this method to replace collapsed conduits, such as those used for sewage, or for other applications. To carry out this process, access space is required at each of the first and second locations. A cable is directed from the second location through the existing conduit back to the first location at which the cable end is connected to an appropriately configured mole. The mole is engaged with a length of a replacement conduit in such a manner that the conduit will follow translatory movement of the mole. At the second location, a cable pulling mechanism is employed. The cable pulling mechanism, which is commonly hydraulically actuated, is braced against the composition and operated to draw the mole through the composition from the first location to the second location. The operator, who is situated at the second location, must monitor the advancement of the mole and disable the cable pulling mechanism at the appropriate time to prevent the mole from detrimentally contacting any part of the cable pulling mechanism and/or its support structure.
One known cable pulling mechanism is disclosed in U.S. Pat. No. 6,305,880. In that mechanism, the mole is advanced, through repeated pulling strokes, over its entire travel path between the first and second locations. In a typical pulling cycle, the mole will be advanced on the order of four inches. Each successive cycle must be initiated by an operator. While this system has been commercially successful, it has a number of inherent drawbacks.
First of all, as a result of the stepwise application of the pulling force, the mole, and following conduit, come to rest each time the mechanism is at the dwell stage for a pulling cycle. As this occurs, the stationary mole and conduit may become temporarily lodged before the pulling force can be reapplied thereto. To reinitiate movement of the mole, a greater force may be required than would be if the mole movement were not interrupted. This places greater demands on the cable pulling structure, the cable, the mole, the conduit, and the structure operatively connecting the mole to the cable and conduit. As a result, there is the potential for premature failure of one or more of these components and a potential reduction in the anticipated life of the overall system.
To address this problem, the system components may be made with increased capacity to ensure reliable operation and an adequately long life for the equipment. This may significantly increase the overall system costs which may have to be passed on to the system purchaser.
Another problem with the above prior art system is that the cable pulling mechanism is required to have potentially a large number of components to coordinatingly interact to alternatingly apply and release the pulling force on the cable. Generally, the more complicated systems become, the more prone they are to malfunction. Further, complicated systems are inherently more expensive than their simpler counterparts.
Still further, the above system has the drawback that the process for moving the mole between the first and second locations may be time consuming by reason of the stepwise advancement of the mole. These types of systems are generally designed to advance a mole through relatively dense compositions that offer a high resistance to movement of the mole. In some environments, such as in loose soil or a preexisting passageway, a significantly lesser resistance to mole movement may be encountered. However, the system operator is nonetheless required to operate the system in the same manner, initiating each advancing cycle, so that the mole moves at a relatively slow rate from the first location to the second location. Since these systems may require two or more individuals to set them up and monitor their operation, the number of man hours required to complete a job may be significant.
To the knowledge of the inventors herein, the industry has not utilized a cable pulling mechanism that is operable to continuously exert a pulling force on a mole to form or enlarge a passageway through a composition, as described above. Even had the industry looked in this direction, as a practical matter there has been lacking a compact unit with the required cable pulling capabilities that could be transported to different sites.
At some sites, the unit is required to be set up for operation underground or otherwise in a restricted space. In these tight quarters, a continuous cable pulling mechanism represents a particular challenge to designers. With a conventional capstan configuration, a V-shaped groove is formed continuously around an annular element that is driven around an axis. The surface bounding the groove and cable are relatively configured and dimensioned so that the cable wedges into the groove surface. This wedging action produces a traction force that causes the cable to follow movement of the annular capstan element around the axis.
Typically, capstans are made with a groove with a uniform cross-sectional configuration around the entire circumferential extent thereof. A cable to be pulled is wrapped around the capstan within the groove so that the center line of the wrapped cable resides in a single plane that is orthogonal to the rotary axis for the annular capstan element. The magnitude of the traction force is dictated by the degree of wrapping of the cable around the capstan. The cable is typically wrapped through significantly less than 360° around the capstan so that there is no interference between the cable at locations at which it initially contacts the capstan and departs therefrom.
The traction force may be increased by increasing the diameter of the capstan so as to increase the contact area between the annular capstan element and the wrapped cable. However, given typical space constraints, there are limitations placed on the dimensions of the capstan.
An alternative way to increase the contact area between the cable and capstan is to cause wrapping through in excess of 360° around the capstan. This requires the formation of a spiral groove pattern on the annular capstan element. As a result, the axial dimension of the capstan increases, as does potentially the cost of the capstan manufacture. Additionally, the use of multiple turns of cable on a capstan causes complications for the system operator. The operator must manipulate the cable around the capstan in a manner that may be inconvenient or impractical in quarters that are close. Further, the inherent stiffness of a cable may make it difficult to produce the multiple wraps and to maintain the cable in the groove in a proper manner preparatory to startup.
Designers of this type of system strive to devise portable systems that can be economically manufactured, will reliably perform in potentially severe environments, can be conveniently and efficiently set up, operated, and broken down, and will perform reliably for an adequate -lifetime. In the interest of economy, it is also a goal for designers of these systems to avoid the unnecessary expenditure of man hours for their operation.